Building a 915, diff is misaligned.

[MichiganMat]

Gentleman,

Im working on building a 915 that we recently acquired and I've run into an issue. When installing the diff, the ring and pinion
seem to meet way before the inner bearing has fully seated in the inner race. The coverplate won't fully close, things don't fit.


Notes on the transmission:

• Standard 915 open diff
• had original bearings with a spacer ring installed for each bearing, as such:

Inner Bearing - spacer - dddddddiiiiiifffffffff - spacer - Outer Bearing



Possible solution 1: move the spacer ring from the outer bearing to the inner as such:

Inner Bearing - spacer - spacer - dddddddiiiiiifffffffff - Outer Bearing

This config would allow the inner bearing to seat properly before the R&P come into contact, BUT Im concerned that the
output flanges would not properly seat or the inner flange would seat too deeply into the case.


Possible solution 2: machine the ring or the diff by ~4mm (the width of the spacer ring)

This solution would keep the original bearing/spacer setup and not misalign the diff in the housing, but Im concerned that
its a "brute force" option that could weaken either the ring or the diff, maybe even causing the diff bolts to not fully seat.


Possible solution 3: the R&P are mismatched and I'll need to find a matching set

Ring #s: 10.75 / 213 / 0.15 - 31 teeth
Pinion #s: 11.75 / 213 / N33 - 8 teeth

Which of the numbers on the diff need to match?


Possible solution 4: The diff is incorrect. Is there a difference between 7:31 and 8:31 diffs? Could it be a 901 diff?

???


Thanks in advance,
Mat

[Matt Monson]

You need a spacer between the ring gear and the differential.

[MichiganMat]

Originally Posted by Matt Monson

You need a spacer between the ring gear and the differential.

I need to move the ring further away from the pinion, not closer.

[daleflesburg]

This is not a do it yourself project. The ring and pinion I believe, are made as sets and are machined to work together. You should not mix them. The depth of the pinion into the ring and the backlash are very critical settings. The proper gauges to set them cost several thousand dollars. You need to send the transaxle to someone who knows what they are doing or you will end up with a box full of junk in a few thousand miles. My son, John Flesburg, could advise you or could do it. He has the necessary tools, the question is does he have the time to do it?

[MichiganMat]

Originally Posted by daleflesburg

This is not a do it yourself project. The ring and pinion I believe, are made as sets and are machined to work together. You should not mix them. The depth of the pinion into the ring and the backlash are very critical settings. The proper gauges to set them cost several thousand dollars. You need to send the transaxle to someone who knows what they are doing or you will end up with a box full of junk in a few thousand miles. My son, John Flesburg, could advise you or could do it. He has the necessary tools, the question is does he have the time to do it?

Totally agreed. Im actually more looking for information as to why these parts are not playing well together. Are they mismatched? What do the numbers mean?

[Daves911L]

The good news is your R&P is a matched set (the "213" match number). The bad is news is something is seriously out of whack and daflesburg is right. You don't move the ring closer or nearer to the pinion. You put it in the correct location in the housing with spacers and the correct measurement tools. Then you move the pinion in or out to the correct engagement depth (the other numbers on your R&P), again using spacers and the correct ($expensive$) measuring tools. My hunch is this box started as a pile of parts and there are missing pieces (spacer and/or shims) on the pinion shaft causing it to sit too far into the differential housing.

[Peter Zimmermann]

My 915 Tutorial should help;

Welcome to Red Line Porsche Wiki - Porsche Wiki

Scroll left side of page, click on "915" under "page tags." The site will take you to the 9-part Tutorial.

"213" is your pair number, which indicates a matched set.

[Jon B]

Originally Posted by MichiganMat

I need to move the ring further away from the pinion, not closer.

You may have an 8:31 ring gear mounted to a 7:31 differential.

[MichiganMat]

Originally Posted by Werkstatt

You may have an 8:31 ring gear mounted to a 7:31 differential.

Hmm yeah, I was *just* thinking that. I'll add it to the original post.

Is there a way to tell the diff's apart? Should I look for a part number and try and run it?

[MichiganMat]

Originally Posted by Peter Zimmermann

My 915 Tutorial should help;

Welcome to Red Line Porsche Wiki - Porsche Wiki

Scroll left side of page, click on "915" under "page tags." The site will take you to the 9-part Tutorial.

"213" is your pair number, which indicates a matched set.

Great, thanks Peter!

[Jon B]

Originally Posted by MichiganMat

Hmm yeah, I was *just* thinking that. I'll add it to the original post. Is there a way to tell the diff's apart?

As a courtesy to people who have already responded, you shouldn't update your original post with new thoughts or information, it can make their answers look foolish.

The 8:31 differential flange is offset 2.0mm further from the pinion, as the ring gear is larger in this direction. The fact that your diff has no provision for a speedo ring indicates it's pre-'76, and very likely a 7:31 version. Maybe Matt Monson can post his graphic with diff dimensions.

[Peter Zimmermann]

Originally Posted by Werkstatt

As a courtesy to people who have already responded, you shouldn't update your original post with new thoughts or information, it can make their answers look foolish.

Great suggestion! Thank you, Jon!

[Matt Monson]

As a courtesy to people who have already responded, you shouldn't update your original post with new thoughts or information, it can make their answers look foolish.
The 8:31 differential flange is offset 2.0mm further from the pinion, as the ring gear is larger in this direction. The fact that your diff has no provision for a speedo ring indicates it's pre-'76, and very likely a 7:31 version. Maybe Matt Monson can post his graphic with diff dimensions.

Jon, correct me if I am mistaken, but the relatively rare 8:31 r/p sets from the mid 70's that used the mechanical speedo drive I believe had the same ring gear flange thickness as a 7:31.

My original comment was based on a guess that he has a fine spline open diff from an SC or 3.2 and his 1975 date stamped ring and pinion is too skinny.

We make a spacer ring that goes between the diff and back of the ring gear that allows one to put a 7:31 r/p set into a late box with the diff flange out farther towards the sidecover.

Or maybe I am completely misunderstanding the issue here but the pictures suggest that the diff needs to be moved more towards the side cover and with fewer shims under the bearing.

I've had the flu and my computer is turned off. I can post some drawings with measurements tomorrow when I am back in the office.

In the meantime, original poster, would you post the part number and prodate off your differential along with whether it is course or fine spline. Is should be on the edge of the ring and will be an 11 digit number and the prodate will have a format like the ones you posted for your ring and pinion ( 10/75 and 11/75 respectively). As others have said your r/p is a matched set.

[Jon B]

Originally Posted by MichiganMat

When installing the diff, the ring and pinion seem to meet way before the inner bearing has fully seated in the inner race. The coverplate won't fully close, things don't fit.

Matt M,

Dates on the R&P indicate they're 1976 model year parts, for transmissions with electronic speedo sensors. The 8:31 ratio began in '75 m.y. on US models, with ROW being 7:31. I'm not aware of a '75 m.y.-only 8:31 offset, could be, but these are not '75 m.y. gears. The orignal quote above suggests to me a ring gear and diff mismatch. He stated the diff had "a spacer ring for each bearing", no speedo disc or doubled-up spacer. I suspect he cannot close the coverplate because the diff offset is incorrect for the ring gear, based on the info provided (so far).

(Sorry to hear you have the flu)

[MichiganMat]

Heres the original ad for the trans: FS: '75-'76 915 W MECH SPEEDO, COOLER & 8.31 (SF Bay Pickup)

From the ad:

The transaxle is stamped 915/40/11 and was an original LSD box. I had to take out the LSD for another project and it is not included in this sale.

915/40 would indicate a '75, BUT the serial stamped on it is 716794* where the 6 indicates 1976. So... I'm not sure if this is a '75 or '76.

Magnesium housing
Mechanical speedo tail housing
8.31 R&P
Electric pump, scavenge filter & cooler setup with RSR style spraybar and pinion squirter (seen in photos)
Carrier included
(I also have a course spline open dif and pair of stub axles that can be negotiated, if wanted)

We took the course spline diff... hrm.

Pics of the part numbers and the flanges:

[Geary]

Yes, the first 6 months of 8:31 production were of unique dimension, with mechanical speedo, and with a unique differential (offset 2.5mm).

You need this unique offset differential, and you need the expertise to determine diff shims "from scratch" (even though pinion depth is already set).

[MichiganMike]

Hi Gears, thanks for the info! So a unique diff is needed in order to use this trans? How would an aftermarket LSD work for this, do they make these special offsets particular for this trans or ?

[Matt Monson]

Sounds like I nailed it. And yes, we have an LSD that could be made to work with what you have there.

Regards,

Matt Monson
Guard Transmission llc.

[Jon B]

Originally Posted by MichiganMat

"fs-75-76-915-w-mech-speedo-cooler-8-31-sf-bay-pickup"

This is an odd transmission, with a '76 serial number but a '75 code. The case has provision for electronic speedo sensor, but is a mechanical speedo drive. Maybe built in '76 for an earlier car, or maybe '76 ZF's didn't have sensor discs.

Assuming Paul's correct about 2.5mm offset, I don't see a problem using 2.0mm offset differential and adjusting bearing shims for 0.5mm (0.020") difference.

I should clarify earlier comment about 8:31 ring gear, the larger pinion head is also a factor in different offset.

[Peter Zimmermann]

Just a comment. Not sure who installed those lock plates...



...but I would much prefer to see them locked down like this: